Semiconductor light emitting element and method for fabricating the same

ABSTRACT

A semiconductor light emitting element includes a substrate  11  having a defect concentrated region  11   a  which has a crystal defect density higher than in the other region. On the substrate  11 , a semiconductor layer  12  is formed. On the defect concentrated region  11   a , a first electrode  13  is formed. On the semiconductor layer  12 , a second electrode  14  is formed.

TECHNICAL FIELD

The present invention relates to a semiconductor light emitting elementand a method for fabricating the same and, more particularly, to asemiconductor light emitting element formed on a substrate having adefect concentrated region with a high crystal defect density and amethod for fabricating the same.

BACKGROUND ART

A semiconductor light emitting element includes a semiconductor layerhaving at least an n-type layer, a light emitting layer, and a p-typelayer which are formed on a wafer. Preferably, the wafer on which thesemiconductor layer is formed is free of crystal defects, and hasexcellent crystallinity. As a method for reducing crystal defects in awafer, a method has been known which forms a region called a crystaldefect region (core) in the wafer. The core is a region which has acrystal defect density higher than in the other region, and is formed toextend through the wafer. By forming the core in the wafer, it ispossible to concentrate the crystal defects in the core. Byconcentrating the crystal defects in the core, a region free of crystaldefects and having excellent crystallinity is formed around the core. Byforming the semiconductor layer of the semiconductor light emittingelement over the region with excellent crystallinity, which is otherthan the core of the wafer, the light emitting element having excellentcharacteristics can be realized.

For example, Patent Document 1 discloses anitride-compound-semiconductor light emitting element which is formed byusing a wafer made of gallium nitride (GaN) and having a plurality ofperiodically formed cores. The nitride-compound light emitting elementdisclosed in Patent Document 1 uses the wafer in which the plurality ofcores are periodically arranged, and regions each having excellentcrystallinity are formed between the cores. By forming ridge stripes inthe regions of the wafer with excellent crystallinity, the semiconductorlight emitting element utilizing semiconductor layer with excellentcrystallinity is realized. In addition, by forming electrodes in theregions other than the cores, a current is prevented from flowing in thecores. This prevents an increase in leakage current due to the cores.

Patent Document 1: Japanese Laid-Open Patent Publication No. 2003-229638DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

However, when the electrodes are formed in the regions other than thecores, the portions of the cores become completely useless to cause theproblems of a reduced number of semiconductor light emitting elementsobtainable from a single wafer, and lower production efficiency.

An object of the present invention is to solve the conventional problemsdescribed above, and allow the realization of a semiconductor lightemitting element having electric characteristics which are lesssusceptible to degradation due to a defect concentrated region.

Means for Solving the Problems

To attain the object mentioned above, the present invention provides asemiconductor light emitting element with a structure in which oneelectrode is formed in a region located over a semiconductor layer andover a crystal defect region (core).

Specifically, a semiconductor light emitting element according to thepresent invention includes: a substrate having a defect concentratedregion which has a crystal defect density higher than in the otherregion thereof; a semiconductor layer formed on the substrate; a firstelectrode formed on the defect concentrated region; and a secondelectrode formed on the semiconductor layer.

In the semiconductor light emitting element of the present invention,the first electrode is formed on the defect concentrated region so thata current flowing from the second electrode to the first electrode flowsfrom the entire second electrode to the first electrode through thesemiconductor layer. Therefore, the current does not flow in the defectconcentrated region within the substrate so that the occurrence of aleakage current in the defect concentrated region is prevented. As aresult, even when the substrate contains the defect concentrated region,the electric characteristics of the semiconductor light emitting elementare scarcely adversely affected thereby. In addition, the defectconcentrated region does not become useless.

In the semiconductor light emitting element of the present invention,the semiconductor layer may include an n-type layer, a light emittinglayer, and a p-type layer which are formed successively on thesubstrate, the first electrode may be formed on the n-type layer, andthe second electrode may be formed on the p-type layer.

In the semiconductor light emitting element of the present invention,the defect concentrated region may be formed either in a peripheralportion of the substrate or in a center portion of the substrate. Inthis case, the peripheral portion of the substrate is preferably acorner portion of the substrate.

In the semiconductor light emitting element of the present invention,the substrate is preferably a cut out portion of a wafer in which thedefect concentrated regions are periodically arranged.

A method for fabricating a semiconductor light emitting elementaccording to the present invention includes the steps of: preparing awafer in which a plurality of defect concentrated regions areperiodically arranged; forming a semiconductor layer on the wafer;forming a first electrode on each of the defect concentrated regions;and forming a second electrode on the semiconductor layer.

Since the method for fabricating a semiconductor light emitting elementof the present invention uses the wafer in which the plurality of defectconcentrated regions are periodically arranged, the alignment of thefirst electrode is easy. Accordingly, the production efficiencyimproves. In addition, it is possible to increase the number ofsemiconductor light emitting elements obtainable from a single wafer.

EFFECT OF THE INVENTION

In accordance with the present invention, it is possible to realize asemiconductor light emitting element with electric characteristics whichare less susceptible to degradation due to a defect concentrated regionwithout lowering production efficiency.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B show a semiconductor light emitting element according toa first embodiment of the present invention, of which FIG. 1A is a planview and FIG. 1B is a cross-sectional view along the line Ib-Ib;

FIGS. 2A to 2D are cross-sectional views illustrating a method forfabricating the semiconductor light emitting element according to thefirst embodiment of the present invention in the order of process steps;

FIGS. 3A and 3B are cross-sectional views illustrating the method forfabricating the semiconductor light emitting element according to thefirst embodiment of the present invention in the order of process steps;

FIGS. 4A and 4B are cross-sectional views illustrating the method forfabricating the semiconductor light emitting element according to thefirst embodiment of the present invention in the order of process steps;

FIGS. 5A and 5B are cross-sectional views illustrating the method forfabricating the semiconductor light emitting element according to thefirst embodiment of the present invention in the order of process steps;and

FIGS. 6A and 6B show a semiconductor light emitting element according toa second embodiment of the present invention, of which FIG. 6A is a planview and FIG. 6B is a cross-sectional view along the line VIb-VIb.

DESCRIPTION OF NUMERALS

-   -   11 Substrate    -   11 a Core    -   12 Semiconductor Layer    -   12 a Defect Concentrated Portion    -   13 n-Side Electrode    -   14 p-Side Electrode    -   15 Wafer    -   16 Epitaxial Layer    -   19 Adhesive Sheet    -   121 n-Type Layer    -   122 Light Emitting Layer    -   123 p-Type Layer    -   141 p-Side Electrode Material    -   171 Mask Pattern    -   172 Resist Pattern    -   181 Wax    -   182 Ceramic Disc

BEST MODE FOR CARRYING OUT THE INVENTION Embodiment 1

A first embodiment of the present invention will be described withreference to the drawings. FIGS. 1A and 1B show a semiconductor lightemitting element according to the first embodiment of the presentinvention, of which FIG. 1A shows a plan structure thereof and FIG. 1Bshows a cross-sectional structure thereof along the line Ib-Ib;

As shown in FIG. 1, a semiconductor light emitting element according tothe first embodiment is formed on a substrate 11 having a defectconcentrated region (core) 11 a where crystal defects are moreconcentrated than in the other region thereof. In the presentembodiment, the substrate 11 is a single-crystal substrate made of anitride semiconductor such as gallium nitride (GaN), and shaped as acube having sides each 1000 μm long, and a thickness of 300 μm. The core11 a extends through the substrate 11 in the thickness directionthereof, and is formed in the corner portion of the substrate in thepresent embodiment.

On the substrate 11, a semiconductor layer 12 is formed. Thesemiconductor layer 12 has an n-type layer 121, a light emitting layer122, and a p-type layer 123 which are formed successively on thesubstrate 11.

The n-type layer 121 is made of GaN, aluminum gallium nitride (AlGaN),or the like having a thickness in a range of 0.5 μm to 10 μm, and has ann-type conductivity. It is also possible to provide a buffer layer madeof GaN, indium gallium nitride (InGaN), or the like between the n-typelayer 121 and the substrate 11.

The light emitting layer 122 has a multiple quantum well structure inwhich well layers each made of InGaN or the like having a thickness in arange of 0.001 to 0.005 μm, and barrier layers each made of GaN or thelike having a thickness in a range of 0.005 μm to 0.02 μm arealternately laminated. It is also possible to insert an n-typesemiconductor layer containing indium (In) between the light emittinglayer 122 and the n-type layer 122 or into the n-type layer 121.

The p-type layer 123 is made of AlGaN, GaN, or the like having athickness in a range of 0.05 μm to 1 μm, and has a p-type conductivity.

The portion of the semiconductor layer 12 which is formed on the core 11a forms a defect concentrated portion 12 a in which crystal defects aremore concentrated than in the other portion. In the semiconductor lightemitting element, of the present embodiment, the p-type layer 123, thelight emitting layer 122, and the n-type layer 121 are each partlyremoved in a region including the defect concentrated portion 12 a sothat a recessed portion exposing the n-type layer 121 is formed.

On the exposed portion of the n-type layer 121, an n-side electrode(first electrode) 13 is formed. On the p-type layer 123 forming a mesaportion, a p-side electrode (second electrode) 14 is formed. As aresult, the n-side electrode 13 is formed in the region of thesemiconductor layer 12 which is located over the core 11 a of thesemiconductor substrate 11. On the other hand, the p-side electrode 14is formed in the region of the semiconductor layer 12 which is otherthan the region located over the core 11 a.

The n-side electrode 13 of the present embodiment has an n-contactelectrode and an n-bonding electrode which are formed successively onthe n-type layer 121. For the n-contact electrode, a single-layer filmmade of platinum (Pt), nickel (Ni), cobalt (Co), aluminum (Al), titanium(Ti), or the like, or a multilayer film composed thereof may be usedappropriately. For the n-bonding electrode, gold (Au), Al, or the likemay be used appropriately. In particular, in terms of a bondingproperly, the outermost layer is preferably made of Au. In the presentembodiment, Ti is used for the n-contact electrode, and Au is used forthe n-bonding electrode. It is also possible to insert a barrier layermade of platinum (Pt) or the like between the n-contact electrode andthe n-bonding electrode.

The p-side electrode 14 of the present embodiment has a p-contactelectrode, a reflection electrode, and a p-bonding electrode which areformed successively on the p-type layer 123. By making the p-contactelectrode of Pt having a film thickness of about 0.001 μm, it ispossible to allow the p-contact electrode to retain a hightransmittance, while suppressing a contact resistance. The reflectionelectrode is preferably formed of rhodium (Rh), silver (Ag), an Agalloy, or the like having a high reflectance so as to reflect light fromthe light emitting layer 122 toward the substrate 11. To cause lightreflection, the thickness of the reflection electrode is preferably setto a value in a range of 0.01 μm to 0.5 μm. For the p-bonding electrode,Au, Al, or the like may be used appropriately. In terms of adhesion withthe p-contact electrode and the reflection electrode, it is alsopossible to laminate Au, Al, or the like and a single-layer film made ofTi, chromium (Cr), molybdenum (Mo), tungsten (W), or the like or amultilayer film composed thereof. In terms of a bonding property, theoutermost layer is preferably made of Au. In the present embodiment, thep-bonding electrode is provided with a multilayer structure of Ti andAu. By thus providing a structure as mentioned above, the lightgenerated in the light emitting layer can be reflected by the p-sideelectrode, and extracted from the substrate 11.

The p-side electrode 14 may also be provided with a transparentelectrode structure. In this case, the light generated in the lightemitting layer can be extracted from the p-side electrode 14. In thiscase, a transparent film made of indium tin oxide (ITO) or the like toserve as the p-contact electrode is formed on substantially the entiresurface of the p-type layer 123, and the p-bonding electrode (padelectrode) is locally formed thereon. In the p-bonding electrode, it isappropriate to use Ti or Rh for the first layer, and use Au for thesecond layer.

In the semiconductor light emitting element according to the presentembodiment, a current flowing from the p-side electrode 14 to the n-sideelectrode 13 flows from the entire p-side electrode 14 to the n-sideelectrode 13 through the p-type layer 123, the light emitting layer 122,and the n-type layer 121 without flowing in the core 11 a within thesubstrate 11. This allows the prevention of the occurrence of a leakagecurrent in the core 11 a. Therefore, even when the substrate 11 containsthe core 11 a, the core 11 a scarcely adversely affects the electriccharacteristics of the semiconductor light emitting element.

Additionally, the semiconductor light emitting element according to thepresent embodiment is formed such that the core 11 a is located in theperipheral portion of the substrate 11. As a result, the n-sideelectrode 13 is disposed on the peripheral portion of the semiconductorlayer 12. Accordingly, the area of the portion of the light emittinglayer 122 which should be removed for the formation of a region wherethe n-side electrode 13 is formed is small. Therefore, it is possible toensure a large light emitting area, and intend higher brightness. Inparticular, since the n-side electrode 13 is formed at the cornerportion of the semiconductor layer 12 in the semiconductor lightemitting element of the present embodiment, it is possible to ensure alarge light emitting area, and intend higher brightness by forming thesubstrate 11 into a square plan shape.

Referring to the drawings, a method for fabricating the semiconductorlight emitting element according to the first embodiment will bedescribed hereinbelow. FIGS. 2 to 5 show the method for fabricating thesemiconductor light emitting element according to the present embodimentin the order of process steps

First, as shown in FIG. 2A, a wafer 15 made of GaN is prepared. Thewafer 15 has a plurality of the cores 11 a that are formed periodically.Subsequently, the n-type layer 121, the light emitting layer 122, andthe p-type layer 123 are epitaxially grown successively on the wafer 15to form an epitaxial layer 16 serving as the semiconductor layer 12. Theportions of the epitaxial layer 16 which are formed on the cores 11 aform the defect concentrated portions 12 a in which crystal defects aremore concentrated than in the other portion. Further, a silicon dioxide(SiO₂) film having a thickness of about 0.5 μm is formed on theepitaxial layer 16 using a chemical vapor deposition (CVD) method, asputter method, a vacuum evaporation method, or the like. Thereafter,the SiO₂ film is patterned using photolithography to form a SiO₂ maskpattern 171. At this time, the SiO₂ mask pattern 171 is formed to exposethe defect concentrated portions 12 a.

Next, as shown in FIG. 2B, depressed portions are formed in regionsincluding the defect concentrated portions by partly removing each ofthe p-type layer 123, the light emitting layer 122, and the n-type layer121 from the crystal growth surface of the epitaxial layer 16 using areactive ion etching (RIE) method. After the formation of the depressedportions, the SiO₂ mask pattern 171 is removed by etching.

Next, as shown in FIG. 2C, a resist pattern 172 covering at least thedepressed portions and the peripheries thereof is formed.

Next, as shown in FIG. 2D, a p-side electrode material 141 isvapor-deposited on substantially the entire surface of the wafer byusing the resist pattern 172 as a mask.

Next, as shown in FIG. 3A, the p-side electrodes 14 are formed bylifting off the resist pattern.

Next, as shown in FIG. 3B, after a resist pattern exposing the depressedportions is formed, an n-side electrode material is vapor-deposited onsubstantially the entire surface of the wafer, and then the n-sideelectrodes 13 are formed by lifting off the resist pattern. It is to benoted that the order in which the p-side electrodes 14 and the n-sideelectrodes 13 are formed may also be reversed.

Next, as shown in FIGS. 4A and 4B, grinding and polishing is performedwith respect to the back surface of the wafer 15. In the grinding andpolishing, the back surface of the wafer 15 is oriented upward byplacing the wafer 15 on a ceramic disc 182 coated with wax 181 so thatthe crystal growth surface thereof faces downward. Then, using apolishing apparatus, polishing is performed with respect to the backsurface of the wafer 15 to provide a predetermined thickness andpredetermined surface roughness. As a result, it becomes possible tostably perform the dicing of the wafer 15.

Next, as shown in FIG. 5A, the wafer 15 to which polishing has beencompleted is sticked to an adhesive sheet 19, and scribing is performedby laser scribing. Then, as shown in FIG. 5B, the wafer 15 is divided bybraking into separate chips each having a predetermined configuration.It is also possible to omit grinding and polishing depending on the sizeof each of the chips. To remove an adherent which has occurred duringchip separation, cleaning with an acid and pure water is performed asnecessary. In this manner, the semiconductor light emitting elements inwhich the n-side electrodes 13 are formed on the cores 11 a can berealized.

By thus using a wafer having the periodically formed cores 11 a as thewafer 15 on which the epitaxial layer 16 serving as the semiconductorlayer 12 is to be formed, the positions where the n-side electrodes 13are to be formed can be easily determined in accordance with thepositions of the cores 11 a. Therefore, it is possible to fabricate amaximum number of the semiconductor light emitting elements from thewafer 15 having a limited size.

Embodiment 2

A second embodiment of the present invention will be describedhereinbelow with reference to the drawings. FIGS. 6A and 6B show asemiconductor light emitting element according to the second embodiment,of which FIG. 6A shows a plan structure thereof, and FIG. 6B shows across-sectional structure thereof along the line VIb-VIb. Thedescription of the components shown in FIGS. 6A and 6B which are thesame as those shown in FIG. 1 will be omitted by providing the samereference numerals.

As shown in FIGS. 6A and 6B, the semiconductor light emitting element ofthe present embodiment has the core 11 a formed in substantially thecenter of the substrate 11. Accordingly, the n-side electrode 13 ispositioned at the center portion of the semiconductor layer 12. As aresult, the current from the p-side electrode 14 flows from the entirep-side electrode 14 to the n-side electrode 13 positioned at the centerportion of the semiconductor layer 12 through the p-type layer 123, thelight emitting layer 122, and the n-type layer 121. Therefore, in thelight emitting element of the present embodiment, the current has anexcellent diffusion property so that a drive voltage is reduced. It isto be noted that the core ha need not be positioned at the exact centerof the substrate.

In the semiconductor light emitting element according to the presentembodiment, the core 11 a is formed in a cylindrical shape so that then-side electrode 13 is formed in a circular shape which is slightlylarger than the shape of the core 11 a. When the n-side electrode 13 isformed in a circular shape of the same size as that of the core 11 a,all the currents flowing in the n-side electrode 13 pass through thecore 11 a so that the drive voltage increases undesirably. In thesemiconductor light emitting element of the present embodiment, then-side electrode 13 is formed in the circular shape larger than that ofthe core ha so that the current from the p-type layer 123 flows to then-side electrode 13 without passing through the core 11 a. As a result,it is possible to ensure a high current diffusion property. However,when the n-side electrode 13 is formed excessively large in size, thelight emitting region is reduced so that the size of the n-sideelectrode 13 is preferably determined properly relative to the core 11a. Thus, it becomes possible to enlarge the area of the p-side electrode14, and ensure a larger area for the light emitting region. Although then-side electrode 13 is formed in the circular shape, it may also beformed in a polygonal shape represented by a square shape or a hexagonalshape. The shape of the core 11 a is not also limited to the cylinder.

In each of the first and second embodiments, the n-side electrode 13 isformed on the n-type layer 121. However, it is also possible to exposethe substrate 11 in the recessed portion, and form the n-side electrode13 directly on the substrate 11. Although the example has been shown inwhich the n-side electrode 13 completely overlaps the core 11 a, thereshould be no problem even when the n-side electrode 13 is displaced fromthe core 11 a, and a part of the core 11 a is not covered with then-side electrode 13.

INDUSTRIAL APPLICABILITY

In accordance with the present invention, it is possible to realize asemiconductor light emitting element having electric characteristicswhich are less susceptible to degradation due to a defect concentratedregion without reducing production efficiency. The present invention isparticularly useful for a semiconductor light emitting element formed ona substrate having a defect concentrated region with a high crystaldefect density, and for a method for fabricating the same.

1. A semiconductor light emitting element comprising: a substrate havinga defect concentrated region which has a crystal defect density higherthan in the other region thereof; a semiconductor layer formed on thesubstrate; a first electrode formed on the defect concentrated region;and a second electrode formed on the semiconductor layer.
 2. Thesemiconductor light emitting element of claim 1, wherein thesemiconductor layer includes an n-type layer, a light emitting layer,and a p-type layer which are formed successively on the substrate, thefirst electrode is formed on the n-type layer, and the second electrodeis formed on the p-type layer.
 3. The semiconductor light emittingelement of claim 1, wherein the defect concentrated region is formed ina peripheral portion of the substrate.
 4. The semiconductor lightemitting element of claim 3, wherein the peripheral portion of thesubstrate is a corner portion of the substrate.
 5. The semiconductorlight emitting element of claim 1, wherein the defect concentratedregion is formed in a center portion of the substrate.
 6. Thesemiconductor light emitting element of claim 1, wherein the substrateis a cut out portion of a wafer in which the defect concentrated regionsare periodically arranged.
 7. A method for fabricating a semiconductorlight emitting element, the method comprising the steps of: preparing awafer in which a plurality of defect concentrated regions areperiodically arranged; forming a semiconductor layer on the wafer;forming a first electrode on each of the defect concentrated regions;and forming a second electrode on the semiconductor layer.
 8. Thesemiconductor light emitting element of claim 1, wherein thesemiconductor layer includes an n-type layer, a light emitting layer,and a p-type layer which are formed successively on the substrate, thefirst electrode is formed on the substrate, and the second electrode isformed on the p-type layer.